Copy header from one VCF to another

[multimeric]

My use case is that I'm writing a VCF merge tool. For that reason, I want the output VCF to have the union of all input VCF headers (INFO and FORMAT, mostly). Unfortunately this is tricky to do in cyvcf2. We have add_info_to_header and co, as well as add_to_header. However, add_info_to_header takes a dictionary and not a HREC object we get from header_iter(), nor is there a way to convert an HREC to a dict. On the other hand, add_to_header(str(some_hrec)) seems like it should work, but it just gives me the error Exception: couldn't add '%s' to header. My current example code is:

in_vcf = cyvcf2.VCF(some_path)
out_vcf = cyvcf2.Writer(some_path, tmpl=some_other_vcf)
for field in in_vcf.header_iter():
    if field.type in ('FORMAT', 'INFO') and not out_vcf.contains(field['ID']):
        out_vcf.add_to_header(str(field))

The error I get here is:

  File "/some/path/vcf_transform/vcf_transform.py", line 83, in merge_vcfs
    out.add_to_header(str(field))
  File "cyvcf2/cyvcf2.pyx", line 297, in cyvcf2.cyvcf2.VCF.add_to_header
Exception: couldn't add '%s' to header
python-BaseException

Is there a working way to copy headers from one VCF to the output?

[CholoTook]

Do you need to call str(field)? Shouldn't add_to_header expect a header line object?